The present invention relates to an apparatus and method for automatically selecting an amount of build-out capacitance to be applied to a hybrid in a telecommunications circuit to balance or substantially balance the hybrid. More particularly, the present invention relates to an apparatus and method for measuring the voltages of signals being transmitted on telecommunication lines to which the hybrid is coupled, and based on the measured values, automatically selecting any number of capacitive elements to be coupled to the balance port of the hybrid as the build-out capacitance.
In the field of telecommunications, it is common to use a hybrid function as an interface between the external tip and ring lines of a xe2x80x9cplain old telephone servicexe2x80x9d (POTS) line and any other signal processing circuitry within a unit. That is, the tip and ring lines of the POTS line are capable of handling two-way signal transmission. However, most processing circuitry requires the transmission to be split into one-way paths before the signals can be processed. Such an interface is commonly used in telephones, modems, and so on, to couple traffic coming out of the device into the POTS line (receive as viewed from the POTS line), and traffic from the POTS line (transmit as viewed from the POTS line) into the device.
A hybrid can be a single-transformer type, as shown in FIG. 1, or a dual-transformer type, as shown in FIG. 2. In a telecommunications system using either a single-transformer hybrid or a dual-transformer hybrid, the hybrid couples the tip and ring lines of the POTS line (identified as the xe2x80x9c2-WLinexe2x80x9d in FIGS. 1 and 2) to a pair of transmit lines (xe2x80x9cNTxxe2x80x9d in FIG. 1 and xe2x80x9c4-WTxxe2x80x9d in FIG. 2) and a pair of receive lines (xe2x80x9cNRxxe2x80x9d in FIG. 1 and xe2x80x9c4-WRxxe2x80x9d in FIG. 2), which can be coupled to the transmit and receive ports, respectively, of a telephone, modem, private branch exchange unit (PBX), or the like. For example, the two transmit lines can be coupled to a microphone or transmitter of the telephone, while the two receive lines can be coupled to a speaker or earphone of the telephone. The signal being transmitted by the microphone will appear between the two transmit lines, while the signal being received by the speaker will appear between the two receive lines.
In other words, the signal being transmitted on the transmit lines is represented by the difference in potential between the two transmit lines, while the signal being received on the receive lines is represented by the difference in potential between the two receive lines. Of course, one of the receive lines and one of the transmit lines could be coupled in common to ground. This is generally done in the single-transformer type by grounding the external connection on Zo BAL. In the dual-transformer type, one side of each port is connected to ground to accomplish the same.
The hybrid converts a signal being transmitted on the two transmit lines so that it is transmitted between the tip and ring lines of the POTS line, while the hybrid also converts a signal being received on the tip and ring lines of the POTS line into a signal that is received over the two receive lines. It is again noted that the signals being received and transmitted can appear simultaneously on the tip and ring lines of the POTS line.
When employing a hybrid in the manner described above, it is important that the signal being transmitted over the two transmit lines is essentially isolated from the signal being received over the two received lines, and vice-versa, so that minimal electrical coupling between the paths occurs. Preferably, none of the signal being transmitted over the two transmit lines should appear on the receive lines, and likewise, none of the signal being received on the two receive lines should appear on the two transmit lines.
The degree to which the signals on the transmit and receive lines are isolated from each other is dependent on the trans-hybrid loss of the hybrid. If the trans-hybrid loss of the hybrid is low, some of the signal being transmitted over the transmit lines may appear on the receive lines and vice versa. However, if the hybrid has a very high trans-hybrid loss, the transmit and receive lines will be essentially isolated from each other so that their respective signals do not appear on each other""s lines.
The amount of trans-hybrid loss of the hybrid is affected by the impedance characteristics of the tip and ring lines of the POTS line. In particular, the amount of trans-hybrid loss of the hybrid is influenced by the amount of capacitance that is present at the interface of the hybrid and the tip and ring lines. The amount of capacitance between and the amount of resistance of the tip and ring lines depends on the length of the tip and ring lines, as well as the gauge of wire that is used for those lines. In standard practice, telephone companies use twisted pair cables having about 0.083 xcexcF of capacitance per mile, regardless of gauge.
The capacitance appearing at the interface of the hybrid and the tip and ring lines will tend to place the hybrid xe2x80x9cout of balancexe2x80x9d, which reduces the amount of trans-hybrid loss in the hybrid and thus allows coupling to occur between the transmit and receive lines. However, that capacitance can be compensated for to xe2x80x9cbalancexe2x80x9d the hybrid by adding additional capacitance to a balance port of the hybrid, which is usually configured as a tap on one of the coils of the transformer or transformers in the hybrid.
For example, a circuit card developed by Pulse Communications, Inc., which functions as an interface circuit to a central office switch (e.g., a switching unit to which telephones are connected), includes a hybrid for coupling the tip and ring lines of a POTS line to transmission and receive lines in the manner described above. This circuit card includes a plurality of capacitors, which are each coupled to a manual switching device that enables the capacitors to be selectively coupled to the balance port of the hybrid.
Specifically, the circuit card includes 2 nF, 4 nF, 8 nF, 16 nF, 32 nF and 64 nF capacitors, which are each coupled to the hybrid by a respective switch of the manual switching device. By opening or closing the switches as desired, a capacitance of 0-126 nF in 2 nF steps can be applied to the balance port of the hybrid. Accordingly, an appropriate amount of capacitance can be applied to the hybrid as a xe2x80x9cbuild-out capacitancexe2x80x9d to compensate for capacitance that is applied to the hybrid by the tip and ring lines. For instance, if the installer of the circuit card determines that 10 nF of build-out capacitance is sufficient to compensate for the capacitance that is applied to the hybrid by the tip and ring lines, the switches coupled to the 2 nF capacitor and 8 nF can be selected, so that a total of 10 nF of capacitance will be applied to the hybrid circuit.
This manual switching configuration of the circuit card described above has certain disadvantages. For example, when this type of circuit card is installed in an existing telephone system, a skilled technician must determine the capacitance applied to the hybrid by the tip and ring lines by using a trial-and-error measurement technique. He attempts to maximize trans-hybrid loss by guessing the best setting, activating the appropriate switches, measuring trans-hybrid loss, then adjusting his guess repeatedly. As he zeros in on the best setting, the trans-hybrid loss improves. This measuring and setting process is very time consuming. Therefore, if a large number of circuit cards are being installed, a technician may have to work for many days to complete the installation. Furthermore, error factors associated with the technician""s equipment, the technician""s abilities, and so on will influence the measurements and thus could result in an inaccurate amount of build-out capacitance being selected.
To overcome these disadvantages associated with a manually adjustable circuit card, Pulse Communications, Inc. has developed a circuit card, for use as an interface circuit, that is capable of automatically compensating for capacitance supplied to the hybrid by the tip and ring lines. This circuit card is also capable of automatically compensating for loss due to the resistance present in the telephony equipment, such as a telephone or switch connected to the circuit card.
This automatic type of circuit card includes, in particular, a digital signal processor coder/decoder (DSP CODEC) which is controlled by a microcontroller to balance the hybrid and provide an appropriate gain factor to the signals being transmitted and received in order to compensate for loss in the lines. The microcontroller accomplishes these functions by adjusting the feedback coefficients of the DSP CODEC to model a signal transmission and receive path which will compensate for the loss in the lines and balance the hybrid as best as possible. However, the microcontroller and DSP CODEC are complicated devices and therefore expensive.
Accordingly, a continuing need exists for a simple and inexpensive circuit that is capable of automatically applying an appropriate amount of build-out capacitance to a hybrid used in a telecommunication circuit to balance the hybrid and eliminate crosstalk between the transmission and receive lines coupled to the hybrid.
An object of the present invention is to provide a method and apparatus for automatically applying an appropriate build-out capacitance to a hybrid, used in a telecommunication circuit, to balance or substantially balance the hybrid so that the trans-hybrid loss is maximized and crosstalk between the transmission and receive lines coupled to the hybrid is minimized.
Another object of the invention is to provide a method and apparatus which is capable of determining a parameter of the tip and ring lines of a POTS line to which a hybrid is coupled, and based on that parameter, controlling a switching apparatus coupled to a plurality of capacitive elements having different capacitance values to apply an appropriate build-out capacitance to the hybrid circuit. The switching apparatus is further capable of being controlled manually independent of the determined parameter.
These and other objects of the present invention are achieved by providing an apparatus, adaptable for use with a hybrid that is employed in a telecommunications circuit, comprising a controller for controlling the status of a plurality of switches that are coupled to a plurality of capacitive elements having different capacitance values to selectively apply an appropriate combination of the capacitance elements as a build-out capacitance to the balance port of the hybrid. The controller is capable of automatically determining the appropriate amount of build-out capacitance to be applied to the balance port of the hybrid based on voltages measured on the tip and ring lines of the POTS line coupled to the hybrid during off-hook and on-hook states of the telecommunications circuit. The controller then reads from a memory an appropriate value for the build-out capacitance corresponding to the difference in these measured values, and controls the switches accordingly to couple the appropriate capacitive elements to the balance port of the hybrid.
The apparatus of the present invention is further capable of being switched to a manual mode in which the switches can be manually manipulated, independent of the controller, to couple the capacitive elements to the balance port of the hybrid in any desired combination.